The present invention relates to a semiconductng device and a method for the manufacture thereof, in particular to a photovoltaic cell having two or more selectively diffused regions produced in a single diffusion step.
Many semiconductor devices make use of differently doped regions of the same conductivity type (p or n) to achieve better performance of the device. The formation of these differently doped regions often implies additional process steps that increase the cost of production. A typical semiconductor device 1 is shown in top-view in FIG. 1, which may be a photovoltaic cell, sometimes called a solar cell. A semiconductor substrate 2 of a first doping type has doped surface regions 5, typically of a second doping type, and a metallisation pattern 3 on at least one major surface. The metallisation pattern 3 usually includes elongate metal electrodes 4 between and under which are doped regions 5, e.g. emitter, collector or gate regions. The electrodes 4 may form ohmic contacts to heavily doped regions 6 (shown best in FIGS. 3 and 4) of the underlying substrate 2. Electrodes 4 collect from, or supply electric currents to the regions 5, which may include semiconductor devices. For a solar cell, region 5 is usually the emitter region.
Solar cells generally suffer from losses in the emitter region 5. For optimum performance the doping level in the emitter region 5 should be at a low level for optimised photon collection and conversion whereas the regions 6 should be doped strongly and deeply to give good ohmic contact to the metallisation pattern 4 without shunting the buried junction 8 (see FIGS. 2, 3). Total system cost may be optimised by selecting the best combination of low manufacturing cost with device losses.
A homogenous emitter/collector design for a solar cell as shown schematically in cross-section in FIG. 2 having front metal contacts 4 back metal contacts 9, a diffusion region 7 and a junction 8. The same diffusion region 7 over the whole front side combined with a low-cost metallisation technique such as screen printing of front metal contacts 4 leads to considerable efficiency losses. This is because the diffusion profile of diffusion region 7 (dopant surface concentration and emitter/collector depth from the surface to the junction 8) required to form good ohmic contacts with this metallisation technique is not ideal for the conversion of light to electrical energy in the intermediate emitter/collector regions 5.
A typical homogenous emitter/collector process sequence usually incorporates a structuring and chemical preparation of the wafer surfaces, a diffusion step (in-diffusion of a dopant from the surface/s), an optional oxide passivation step, an optional anti-reflective coating step (e.g. TiO2 or Si3N4) which may provide additional passivation and a metallisation sequence (preferably by screen printing).
An improvement is shown in FIG. 3 in which the regions 5 are etched to a depth such that the selective emitter/collector regions are formed. The depth of the doped layer between the emitter/collector fingers 4 is reduced and hence, the doping level is reduced. Such devices are described in the article xe2x80x9cSimple integral screenprinting process for selective emitter polycrystalline silicon solar cellsxe2x80x9d, by Szlufcik et al., Appl. Phys. Lett. Vol. 59, Issue 13, pp 1583-1584, 1991 and in DE 44 01 782 in which the diffusion areas between the emitters are partially etched after the metal contacts have been protected by a protection layer. Application of the protection layer, typically a polymer paste requires an extra masking step. This latter technique has the disadvantages of an extra masking and a difficult etching step which increase the complexity and cost of manufacture.
FIG. 4 shows schematically in cross-section the more effective solution of a selective emitter/collector which has deep emitter/collector doping profile regions 6 with a high dopant surface concentration under the front metal contacts 3 and a shallow doping profile that is optimised for carrier collection in adjacent areas 5. However, the selective emitter/collector structure requires a more complicated manufacturing process as described, for instance, in DE 42 17 428 in which a shallow emitter layer is diffused over the whole front surface of the substrate followed by the formation of passivation/oxide layers. Openings are then formed in the oxide layers using a laser beam and the oxide layers used as a mask for a deep diffusion step of the emitter contact areas. The metal contacts 3, 4 are then formed in the trenches opened by the laser.
Known manufacturing methods of a selective emitter/collector using low cost metallisation techniques such as screen printing require a second diffusion step and/or masking and/or etching steps resulting in the need for alignment of the metallisation pattern 3 with the higher doped regions 6 on the front side of the semiconductor device 1.
General methods of making photovoltaic devices are described in xe2x80x9cPhysics, technology and use of Photovoltaicsxe2x80x9d by R. J. Overstraeten and R. P. Mertens, Adam Higler Ltd., 1986 which is incorporated herein by reference.
The semiconductor device in accordance with the present invention is defined in claims 1 to 13.
The present invention applies primarily to the formation of two different, selectively diffused regions on semiconductor substrates with different doping levels. The advantageous design of a selective emitter or collector structure is realised without any additional process step or complication of the process in comparison with homogeneous emitter/collector structures. The most favourable process sequence makes use of screen printing a solids based dopant paste to form the diffusion regions by a first high temperature heat treatment step and screen printing a metal paste to provide the metallisation by a second high temperature heat treatment step.
The selective emitter or collector process in accordance with the present invention, for example for a photovoltaic device, may have the same number of process steps as a typical homogenous emitter/collector process and less steps than for a conventional selective emitter/collector process. The method of the present invention provides a simple and economical manufacturing method for photovoltaic devices which have advantageous results over known homogeneous emitter/collector structures. Less dopant source material is required as compared with the homogeneous emitter/collector process, thus reducing the production cost while improving the final cell performance.
The present invention is a simplification of known selective emitter or collector formation process sequences. The selective emitter or collector structure in accordance with the present invention is formed within only one diffusion step. No additional masking and/or etching process steps are needed to form the selective emitter or collector.